EDHF-mediated responses induced by bradykinin in the porcine coronary artery

Hyperpolarization and relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) in the porcine coronary artery involve a cytochrome P450-epoxygenase-derived metabolite. To further characterize the cellular mechanisms underlying EDHF-mediated hyperpolarization and relaxation of the porcine coronary artery, the effects of inhibitors of cytochrome P450-epoxygenase enzymes, potassium channels and gap junctional communication were compared. Sutfaphenazole, a selective cytochrome P450 2C9 inhibitor, attenuated EDHF-mediated hyperpolarizations and relaxations. In the presence of Delta(9)-tetrahydrocannabinol, previously reported to inhibit EDHF production, both EDHF-mediated responses were also blocked to the same extent. The combination of barium plus ouabain had no effect on EDHF-mediated hyperpolarization, but the combination of charybdotoxin plus apamin abolished both the EDHF-mediated hyperpolarization and relaxation. Iberiotoxin, an inhibitor of large conductance Ca2+-dependent K+ channels, and 18alpha-glycyrrhetinic acid, a gap junction uncoupling agent, failed to significantly affect EDHF-mediated responses in bicarbonate-buffered physiological salt solution, but inhibited those recorded in bicarbonate-free solution. These results confirm the crucial role of products of cytochrome P450-epoxygenase in EDHF-mediated responses in the porcine coronary artery. They also indicate that the Na+/K+ ATPase, inwardly rectifying potassium channels and gap junctional communication play little or no role in mediating these responses. Moreover, EDHF-mediated hyperpolarization can be rendered sensitive to iberiotoxin and 18a-glycyrrhetinic acid by the removal of extracellular bicarbonate.